My invention disclosed and claimed herein relates generally to the art of measuring the speed of a jet-propelled watercraft, and more particularly to a speed measuring system capable of determining the speed of a jet-propelled watercraft as a function of the dynamic water pressure produced within the jet intake chamber of such a watercraft.
Traditionally, mechanical speedometers for watercrafts have employed a pitot tube which is mounted to the transom of the boat and linked via a flexible tube to a pressure gauge which is marked with speed graduations for the operator's view. Electronic speedometers, on the other hand, have generally employed a rotating paddle wheel which is electrically linked to an electronic display for the operator's viewing. While the above traditional speedometers have been found useful in measuring the speed of larger watercrafts, their use has not been considered acceptable in connection with smaller personal jet-propelled watercrafts, such as those commonly known as the Jet Ski and Wave Runner.
Because a small personal watercraft is most often boarded from the water, it is specifically designed to utilize a jet drive so as to eliminate the safety hazard of an exposed propeller. In such a personal watercraft, an impeller shaft carrying an impeller extends into a protected jet intake chamber, through which water is drawn for propulsion of the watercraft. Thus, the operator of the watercraft is shielded from the impeller.
Use of a pitot tube in connection with a personal watercraft also raises significant safety concerns. A traditional pitot tube, which is generally mounted exteriorly to the transom of a boat, will oftentimes resemble a fin or dagger which, if mounted to a personal watercraft, poses a significant safety concern for the operator while boarding the watercraft. Additionally, externally exposed pitot tubes are subject to breakage, as personal watercrafts are oftentimes driven or pulled entirely onto a beach where damage to an exposed pitot tube may occur. Traditional paddle wheels, while somewhat safer, are also unreliable, become clogged with dirt, and are subject to breakage when exposed to such conditions.
Theoretically, on jet-propelled watercrafts, the above safety and durability concerns could be alleviated by providing a pitot tube which is not externally exposed. However, as a practical matter, this is a difficult task, since the pitot tube must extend into the water through which the watercraft is traveling, in order to obtain a true water pressure reading for conversion to speed. Moreover, any obstruction tending to interfere with the clean flow of the water will affect and alter the true water pressure, thereby causing significant errors in the calculation of the true watercraft speed.
For the above reasons, mounting a pitot tube within the jet intake chamber of a jet-propelled watercraft has not heretofore been considered, since it is known that the water pressure within the intake chamber varies significantly from that of the water within which the watercraft travels. The speed of the water through the jet intake chamber is accelerated and heavily non-linear in comparison with the actual speed at which the watercraft is moving. Consequently, the water pressure within the jet intake chamber is also heavily non-linear in comparison to that outside the chamber. Moreover, water which is drawn through the jet intake chamber is highly turbulent, and the physical geometry of the jet intake chamber will mask or shadow the pitot fin, thereby introducing additional non-linearities which seriously affect the speed readings. Additionally, due to the surface proximity of the intake chamber at planing speeds for a personal watercraft, a jet intake mounted pitot fin will often break contact with the water (commonly referred to as "drop-out"), thereby leading to inaccurate and erratic speed readings.
With the advent of the personal watercraft, however, the above safety and durability concerns have become increasingly important, and there has developed a distinct need for a means by which the speed of a jet-propelled watercraft may be accurately determined, while also satisfying the above concerns for safety and durability. By mounting the pitot tube within the jet intake chamber of a jet propelled watercraft, the operator will be protected from unsafe operating conditions, and conditions upon which the pitot tube and speed measuring system can become easily damaged. It is with this objective in mind, that I have developed a speed measuring system hereinafter described and claimed for a jet-propelled watercraft.